Watch what is happening before she does it.
It may be that your mother in law is making a confrontational stare as in eye to eye at the cat which the cat is not going to accept as it sounds as if she believes she is dominant. She may even without realising it be showing the cat a lot of teeth. LOL
Many things can cause this, human body language being misinterpreted, a perfume or deoderant, noise, being touched where it is sensitive and more.
Learn to read the cats body language and if you see it is becomeing upset either remiove the offender or the cat to another room.
She ought to be spayed as well as this may well help and there is a product called Feliway that can be purchased at many vets which works as a room diffuser. This product actually mimics the pheromones from the cats jowls and causes a calming effect on the cat as well.
Hoping you get to the bottom of it as the cat is being upset too.